1. Field of the Invention
This invention relates to ball joint gimbal mounted electro-optic (EO) systems, and more particularly to the use of two-dimensional non-contacting electro-magnetic (EM) forces for providing full three-axis steering control.
2. Description of the Related Art
An EO system includes a directional EO element such as a detector or laser source and a gimbal for pointing the directional EO element. Optics are mounted on the gimbal to receive (detector) or transmit (source) an optical beam to or from the EO element. The optics define a field-of-view (FOV) for the EO element about the pointing axis. The gimbal slews the pointing axis, and FOV, in two axes over a larger field-of-regard (FOR). This type of EO system may be used, for example, on aircraft or various types of munitions e.g. missiles, rockets, artillery shells, etc.
As shown in FIG. 1a, the classic method to control two-axis pointing of pointing axis 8 is to control (and measure) rotation separately in each of two axes (Az/El or Yaw/Pitch) through a nested gimbal arrangement 10 in which a first gimbal 12 is mounted on a second gimbal 14. The axes of rotation 16 and 17 of the first and second gimbals are perpendicular to each other such that each axis of the nested gimbal controls one axis of rotation. Gimbal drive motors are configured to mechanically rotate each gimbal about its axis. With a two axis system, the third rotational axis is kinematically constrained by the position of the first two gimbals. For example, a particular Az, El or yaw/pitch angle pair rigidly specifies a unique roll angle. Roll cannot be independently controlled without adding a third gimbal or some equivalent.
As shown in FIG. 1b, another approach uses a ball gimbal joint 20 that includes an inner ball 22 captured within a socket (not shown). A pair of ultrasonic drive motors 26, 28 are placed in direct contact with inner ball 22 to apply rotational forces 30, 32 about contact points along orthogonal axes (e.g. Az and El) to control two-axis pointing of pointing axis 34. The third rotational axis (e.g. roll) is driven implicitly via control of the first two axes. Unlike with nested gimbals, the third axis is not rigidly constrained and can drift over repeated positioning. A third ultrasonic drive motor may be placed in direct contact with inner ball 22 to provide a third control axis. Each ultrasonic drive motor creates torque by causing a traveling wave in annual metallic elastic body to which piezoelectric elements are glued. This wave is created as each point on the annulus to moves in a small ellipse, back and forth along the circumference of the annulus, and up and down along its axis. The direction and speed of the traveling wave is determined by the phase differences of this motion at points along the circumference. At the bottom of each elliptical cycle, different points along the annulus contact the ball and push it circumferentially about the annulus, like twisting a bottle cap. See Masahiko Hoshina et al “Development of Spherical Ultrasonic Motor as a Camera Actuator for Pipe Inspection Robot” The 2009 IEEE/RSJ International Conference on Intelligent Robots and Systems, Oct. 11-15, 2009 St. Louis, USA.
As shown in FIG. 1c, another approach modeled after the human eye uses multiple tendons 40 attached to the surface of the inner ball 42 of a ball joint gimbal. Rather than applying a rotational force about a contact point, as in the first two approaches, this type of controller applies tangential forces along the surface of the ball at each contact point. Tendons can only pull, specifically in the direction of the tendon. Multiple tendons must act together to provide rotation in an arbitrary direction, or even back and forth motion in the same axis (opposing directions). Three tendons can provide full two-axis control, although four are typically used. Similar to the nested gimbal, the third rotational axis is kinematically constrained by these tendons, just not as rigidly (because roll motion is largely perpendicular to the tendon axes). Examples of this approach are provided in U.S. Pat. Nos. 6,326,759 and 7,032,469.
As shown in FIG. 1d, another approach uses linear electro-magnetic permanent magnet (PM) motor technology to apply tangential forces 50, 52 and 54 along predetermines axes to allow a telescope mounted on an inner ball 56 of a ball joint gimbal to be controlled in three dimensions simultaneously. In one example, rotation is realized by 24 single-axis motor segments (4 yaw, 4 pitch, and 8 roll), with pairs of segments symmetrically distributed around the central bearing unit to balance forces. Each of these segments is independent in the sense that the rotor portion of each segment must remain adjacent to its own stator coil through the range of motion. This implies that the motor pole pitch must be large enough to maintain this alignment over the range of motion, including cross-axis motion from the other motors. This limits the motion in this configuration to about +/−3 degrees. The motor is a permanent magnet excited by double sided linear motor segments. The double-sided motor configuration consumes considerable packaging volume, making it inappropriate for smaller systems. But, the SOFIA telescope includes a 2.5 m diameter primary mirror, with a ball joint gimbal large enough to accommodate the motor configuration. See M. Anders et al “A Novel Spherical Linear PM Motor for Direct Driving Infrared Optical Telescope” Institute of Electrical Energy Conversion, pp. 528-530, 1999.